A biker gang visits a monastery where they encounter black-robed monks engaged in worshipping Satan. When the monks try to persuade one of the female bikers, Helen, to become a satanic sacrifice the bikers smash up the monastery and leave. The monks have the last laugh, though, as Helen, as a result of the satanic rituals, is now possessed and at night changes into a werewolf, with dire results for the biker gang.
http://www.imdb.com/title/tt0067972/